(1) Field of the Invention
Polyurethane powder coating materials comprising polyureas, polyesters, and crosslinkers, having less than 15 mol % succinic acid as the polycarboxylic acid component. These powder coating materials have outstanding properties, especially in the context of producing matt coatings.
(2) Description of the Related Art
Coating materials are used widely to provide decorative and utilitarian properties, such as to improve the durability of a material or other useful functional properties, such as stain, solvent and chemical resistance. Coatings are used in a variety of industrial, commercial or consumer products, including in architectural and building materials and in consumer goods and appliances. For a long time there has been increasing interest in powder coating materials which produce a matt surface. Practical reasons drive this interest: among them the higher degree of cleaning and maintenance required by glossy surfaces compared to matt surfaces and safety considerations for applications involving the need to avoid strongly reflecting surfaces. Additionally, the aesthetic properties of a matt surface are demanded for certain applications.
The easiest way to obtain a matt surface is to admix smaller or larger amounts of fillers, such as chalk, finely divided silica or barium sulfate, to the powder coating material in accordance with the extent of the desired matt effect. However, such additions bring about deterioration in the film properties of the coating, such as adhesion, flexibility, impact strength, and chemical resistance.
The addition of substances incompatible with the coating material, such as waxes or cellulose derivatives, does give rise to matting, but slight changes in the course of extrusion lead to fluctuations in the surface gloss. Therefore, the reproducibility of the matt effect is not ensured.
Polyisocyanates containing urea and uretdione groups have become known only in relatively recent times. DE-A 195 46 750 claims reaction products of isophorone diisocyanate uretdione and disecondary diamines as curing agents for producing polyurethane coatings having glossy surfaces.
DE-A 196 30 844 was the first to describe matt polyurethane powder coating materials which comprise as their curing component isophorone diisocyanate uretdiones containing urea groups. They are prepared by reacting uretdiones with water. Intermediates which form during this reaction, with the elimination of carbon dioxide, are primary amines, which react with remaining isocyanate groups to form ureas. The nitrogen atoms of the urea groups are each monosubstituted. Reactions of polyisocyanates with water are difficult to reproduce owing to the formation of by-products.
DE-A 196 37 375 describes polyurethane powder coating materials which are formulated for a matt effect by means of isophorone diisocyanate uretdiones containing urea groups as curing agents. These curing agents are formed by the reaction of isophorone diisocyanate uretdione with disecondary diamines. The disecondary diamines are reaction products of diprimary diamines and maleic or fumaric esters.
DE-A 196 37 377 and DE-A 19816 547 claim polyaddition products containing uretdione groups and urea groups as a curing component for the matting of polyurethane powder coating materials. These curing agents are prepared by reacting isophorone diisocyanate uretdione with diamines having one primary and secondary amino groups. Preparation of the pure diamines is in some cases complex and very costly.
DE 100 42 318 describes novel matt polyurethane powder coating materials which are easy to prepare, with the powder coatings produced from them having good mechanical properties. Matting is brought about by a combination of a polyurea with a polyester that contains succinic acid. As a result, the range of polyesters which can be used is limited.